A method was developed to separate met and leu enkephalin from each other and from the closely related peptides which are their metabolites or potential precursors. This methodology permitted (1) studies of the catabolism of labelled enkephalin in vitro and in vivo and (2) isolation of endogenous enkephalin from rat striatum for quantitative assay. Studies are under way of the formation of enkephalin using labelled precursors. BIBLIOGRAPHIC REFERENCE: Meek, J.L., Yang, H.-Y. T. and Costa, E.: Enkephalin catabolism in vitro and in vivo. Neuropharmacology 16: 151-154, 1977.